120 HYDROSTATIC TRACTOR
New for 1970 is the 120 Lawn and Garden
Tractor. The 120 is a 12 HP Hydrosta-
tic Drive Tractor. This tractor will
fill the gap between the 112 and 140
The 120 has the general appearance of
the 140 HI Tractor and should appeal to
The following service program will cover
the 120 Tractor. There are items which
will be similar to the 140, however, the
140 Tractor has a printed Service Manual
available (SM 2086). The 120 Tractor
will have a separate Service Manual (SM
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Wheel Base '^6 in.
Over-all Length 67-1/2 in.
Over- all Height 43 in.
Over-all Width (Min) 38 in.
(Max) 42-1/2 in.
Wheel Tread (Center-to-Center)
Front (GT-3, GT-5
and GT-6) 31-1/4 in.
Front (GT-4) 29-1/2 in.
Rear (GT-3, GT-4
and GT-6) 30-1/2 or 32-1/2 in.
GT-3, GT-5 and GT-6
Front 16x6.50-8 2-ply
GT-3 Rear 23x8.50-12 2-ply
GT-4 Front 4.80/4.00-8 4-ply
GT-4 and GT-6
Rear 23x8.50-12 2-ply
GT-5 Rear 23x10.50-12 2-ply
GT-3, GT-5 and
GT-6 Front 6 to 16 psi
GT-3 Rear 5 to 10 psi
GT-4 Front 12 to 40 psi
GT-4 and GT-6
Rear 5 to 10 psi
GT-5 Rear 5 to 10 psi
Curb Weight 736 lbs .
- 1 -
The 120 Tractor will be shipped assem-
bled, less the hood and seat (colored
tractors only) and rear tires on the
Assemble the tractor and attachments
according to the PDI instructions in
the Operator's Manual. Be sure to re-
move the battery for activation. Bat-
tery acid is not shipped with the trac-
tor and is available from the parts de-
120 GEN 2
The free wheeling valve is open when
the tractor is shipped from the factory.
With the valve open the tractor can be
moved without starting the engine.
To close the free wheeling valve, turn
the knob counterclockwise about 10 re-
volutions. If the tractor will not
move with the engine running be sure
the free wheeling valve is closed.
120 GEN 3
Be sure the customer has his Operator's
Manual at the time of delivery.
Explain periodic maintenance checks.
Instruct the customer of safe operation
120 GEN 4
- 2 -
Bore and Stroke 3.38x3.25 in.
Displacement 29.07 cu. in.
Speeds Fast - (No load) 3800 rpm
Speeds Idle - (No load) 1200 rpm
Horsepower (Engine Manufacturers
Rating) 12 (? 3600 rpm
Valve Clearance (Intake)
Cold 0.010 in.
Valve Clearance (Exhaust)
Cold 0.020 in.
Crankcase Capacity 3 US pints
Summer SAE 30 (AM 30730)
Winter SAE 5W-20 (AM 30710)
120 ENG 1
There are three requirements for an en-
gine to operate. They are air, fuel and
spark. A deficiency of any one of tkese
will cause trouble.
Air be sure the air filter is ser-
viced properly. Tap filter lightly to
remove dirt. Never use compressed air
or a liquid cleaner of any type to clean
filter as these may damage air filter
and result in damage to the engine. Re-
place filter every 100 hours of service
or sooner if it becomes extremely dirty.
Fuel use only regular grade gasoline,
never use a premium grade gasoline. Main-
tain a clean fuel tank, line and strainer.
Caution customer about the common prac-
tice of contamination with gasoline.
Nothing will interrupt operation faster
than dirty or contaminated fuel.
Spark maintain a clean spark plug.
Proper spark plug gap is .025-inch. If
the electrodes have burned short or have
become pitted install a new spark plug
of the proper size. (Notice not just
any spark plug. )
■k -k *
- 3 -
The serial number plate for the engine
contains several groups of numbers. The
serial number of the engine is the num-
ber which is immediately below the words
serial number on the serial plate. This
number is valuable to the factory and
should appear on warranty claims and
correspondence referring to engine pro-
120 ENG 2
120 ENG 3
Remove the (4) bolts holding the hood
cowls to tlie frame and remove the hood
and grille components as an assembly.
Disconnect the positive and negative
battery cables at the engine.
mm ^^^ ^m ii^^^^l
' ' *5^
Disconnect the wiring harness at the
three prong connector and electro-
magnetic clutch connection. Shut off
fuel valve and remove gas line. Dis-
connect throttle and choke control cables
(Air filter and carburetor back plate
removed for clarity.)
Remove the two bolts holding the flex
coupling to the drive shaft. Remove the
(4) bolts connecting the engine to the
frame and lift the engine up and out of
For assembly, reverse the removal proce-
The 120 Tractor will have its own Ser-
vice Manual. Until the time you receive
your Service Manual, you may refer to SM
2086 140 Hydrostatic Tractor for most
of the engine disassembly and reassembly
procedures, trouble shooting and diag-
120 ENG 4
- 4 -
NOTE: Insert throttle cable in the lower
of the two holes on the adjusting screw lever.
Raise the control lever to the fast position.
To properly position throttle cable, hold
the adj lis ting screw lever rearward against
the stud while tightening the clip on the
120 ENG 5
To adjust the governor, set the throttle
cable as previously described. Start the
engine and raise the control lever to the
fast position. Turn the governor adjust-
ing screw clockwise to decrease maximum
rpm. Turn the governor adjusting screws
counterclockwise to increase maximum rpm.
Maximum rpm is 3800 no load and idle
speed is 1200 rpm. Shown is a vibrating
tachometer available from Tecumseh,
(Part No. 670156, current price $3.95.)
which will enable you to set the engine
120 ENG 6
- 5 -
FUEL SYSTEM 1
Type of Fuel Regular Gasoline
Filter In-line Strainer
Fuel Tank Capacity .. 1.75 US Gallons
Type AM31400 Dry Filter
Idle adjustment and high speed adjust-
ment must be made at the same time as
each affects the other. Adjust as fol-
1. Be sure choke lever is down
(choke open) . Turn high speed
mixture needle (A) clockwise
until closed. Close finger
tight only. Then open one-
and-one half turns.
2. Turn idle mixture screw (B)
clockwise until closed. Close
finger tight only. Then open
two complete turns.
3. Start engine and raise throttle
lever on dash panel to "fast"
position. Allow engine to warm-
4. Turn high speed mixture needle
(A) 1/8 turn each time, clockwise
or counterclockwise until engine
runs smoothly at full throttle.
Keep needle position slightly on
the rich side (open) when opera-
ting tractor with power-driven
equipment such as the mower or
snow thrower .
120 FS 1
Move throttle lever to "slow"
position and turn idle mixture
screw (B) 1/8 turn each time,
clockwise or counterclockwise
until engine idles smoothly.
Advance throttle lever quickly
to check for uniform acceleration.
If engine misses, gas-air mixture
is too lean. Turn high speed mix-
ture needle counterclockwise until
positive acceleration can be ob-
- 6 -
If excess exhaust smoke is noticed, mix-
ture is too rich. Readjust idle mixture
screw if necessary, until good balance
is achieved and engine idles smoothly
between 1200 and 1700 rpm. The idle stop
screw (C) adjusts the speed at which the
engine idles. This is factory-adjusted
and will not normally require adjustment.
Carburetor and fuel tank and lines ser-
vice is shown quite well in SM 2086.
FUEL SYSTEM 2
* * * A
- 7 -
Battery - JOHN DEERE AM 31186
40 amp-hr. , 12-volt
Starter Switch Ignition Key and
Spark Plug gap 0.025 in.
Breaker Point Gap 0.020 in.
Voltage Regulator Fuse SEE 14 amp.
120 ELECT 1
The 120 Tractor has battery coil ignition.
Shown is the wiring diagram of the 120.
If problems are encountered with the
cranking system, the ignition switch,
solenoid and neutral start switches
should be checked to determine if they
are working properly. Test procedures
are outlined in the 120 and 140 Service
Checking Specific Gravity
To determine whether the battery is ca-
pable of meeting the requirements of the
starting motor, it is necessary to du-
plicate operating conditions by subject-
ing the battery to a load test. To ob-
tain a true test, the battery should be
at least 75 percent charged. This can
be determined by taking a hydrometer
The following table illustrates typical
ranges of specific gravity (amount of
unused sulfuric acid remaining in the
solution) for a cell in various states
of charge, with respect to its ability
to crank the engine at 80° F. with ini-
tial full-charge specific gravity at
either 1.260 or 1.280.
120 ELECT 2
- 8 -
1.260 to 1.280
1.230 to 1.250
1.200 to 1.220
1.170 to 1.190
1.140 to 1.160
1.110 to 1.130
Checking Battery Voltage
There are two methods of testing battery
capacity. Battery electrolyte tempera-
ture should be at or near 80 degrees
Fahrenheit for these tests.
Before making either of the two follow-
ing tests, first check electrolyte level
in battery. Add water if necessary.
If water is added, be sure it is thor-
oughly mixed with the underlying elec-
trolyte by charging. Battery voltage
should be 11.5 to 12.6 volts before
testing. Refer to instructions supplied
by test equipment manufacturer, when
using high-rate equipment.
Using the first method, crank the engine
for 15 seconds with the starting motor
and measure the battery voltage. If
voltage is less than 9.6 volts at the
end of 15 seconds replace battery.
120 ELECT 3
* * * * *
As a second method, use heavy load dis-
charge test equipment.
Discharge the battery according to the
manufacturer's recommendations for test-
If after testing the battery voltage is
less than 9.0 volts, the battery fails to
meet the load test, indicating loss of
capacity or internal short circuits. Any
battery that passes the load test is a
good battery and can be relied upon to
fulfill the requirements of the starting
motor under normal conditions.
120 ELECT 4
- 9 -
Repair to the starter is limited to the
brushes, end cap, armature and starter
drive assembly. Any of the parts can
be replaced if found to be defective.
Field coils in this starter are perma-
nent magnets and no service is required
or possible. Any serious defect in the
magnets will require a complete starter
Disconnect negative cable from battery.
Disconnect solenoid-to-starter cable
from starter terminal.
Remove two mounting bolts holding
starter to engine block.
120 ELECT 5
A * A
Starter Drive Service
Remove starter drive by holding drive
gear and turning 3/8-inch lock nut off
armature shaft. After lock nut has
been removed the three parts of the
drive can be removed.
120 ELECT 6
120 ELECT 7
Remove the two thru bolts which hold
the end caps to starter housing. Re-
move 1/4 X 5/8-inch capscrew from rear
brace. Tap rear brace downward to pro-
vide room for removal of end cap.
Remove the end cap carefully to avoid
loosing brush springs which will pop
out when end cap is removed.
- 10 -
Replace brushes whenever they show any
appreciable amount of wear. To replace
input brush assembly, remove nuts and
washers from terminal and pull out thru
the inside of cap. The input brushes
are to be placed into the insulated
brush holders when assembling.
The leads from the ground brushes are
attached to the metal screws which se-
cure the insulated brush holders to the
end cap .
A * *
The preparation of the starter end cap
is the first step in the assembly of the
First place brush springs into brush
holders, then in succession place a
brush in its respective holder with the
beveled side up.
Compress brush springs and place U-shaped
clips made of banding steel onto each
Wipe commutator clean with a dry cloth
and lubricate armature shaft with a
small amount of light grease.
Place armature into end cap, and retove
brush retaining clips.
120 ELECT 8
* * * * *
Place starter housing over armature
while holding down pressure on the arma-
ture. This prevents the brush springs
from pushing armature up and away from
end cap. If this occurs, re-load brushes
into end cap and reassemble.
Align housing to end cap using the index
marks on cap and housing as a guide.
NOTE: The starter housing has a mark
and an indentation on the inside which
must fit into the two indentures on the
end cap .
P* '" l^f T^
- 11 -
Place steel spacer washer onto armature
shaft and insert armature shaft through
drive end bearing of mounting bracket.
Insert thru bolts and torque to 20 to
Position rear brace and install 1/4 x
5/8-inch capscrew and tighten firmly.
Install starter drive gear on armature
shaft, anti-drift spring, spring cup
and lock nut. Torque lock nut to 45 to
120 ELECT 10
Install starter to engine, using the two
shoulder bolts previously removed. At-
tach solenoid to starter wire and tighten
all bolts firmly.
NOTE: A thin steel shim is used with
some early model starters between mount-
ing bracket and engine. Re-install shim
whenever original mounting bracket is
- 12 -
The battery ignition system differs from
the magneto ignition in two ways.
1, Current is supplied to the ignition
coil from the battery rather than from
a permanent magnet self-powered magneto.
2. The ignition switch must be closed
in the battery system for the current to
flow through the coil. Magneto systems
require an open circuit type switch.
The function of any ignition system is
the same, and that is to produce a hot
spark at the plug at the right time for
ignition of the fuel mixture.
Here is how that battery ignition works:
With the ignition switch closed, current
flows from the battery to the primary
windings of the coil, and through the
closed breaker points to ground.
120 ELECT 11
At the point when the piston is near
top dead center on the compression stroke,
the breaker points are opened by the cam
and push rod.
When the points are opened, the primary
electric circuit is broken and the mag-
netic field breaks down.
With the collapse of the primary field,
the magnetic lines of force cut across
the conductor coil windings and a high
voltage is induced in the secondary
A spark occurs at the spark plug, which
is in the secondary circuit. The high
voltage developed in the secondary coil
causes the current to jump across the
spark plug electrode gap, thus making a
The residual current in the primary
winding is absorbed by the condenser.
This eliminates arcing at the points
and aids in producing a stronger spark
at the spark plug.
- 13 -
The ignition coil is either in a satis-
factory condition or it is not. Coil
failure occurs all at once, much as an
electric light bulb. It does not degen-
When coil failure is suspected, use an
analyzer to test coil. The analyzer will
also test the condenser and solenoid as
well as checking voltage and amperage.
See equipment manufacturers operators
handbook for specifications for parti-
cular unit being tested.
Follow manufacturer's recommendations to
test the following:
1. Coil power test
2. Coil high speed test
3. Coil surface insulation test
4. Coil continuity test
5. Coil ground test
120 ELECT 12
The condenser can be tested with an ohm-
meter. To do this, remove the condenser
lead from the breaker points, then con-
nect tester leads between the condenser
lead and a good ground on engine. At
first, a low resistance should be indi-
cated. However, this should very quickly
rise to a high value. If low resistance
is indicated continuously, then conden-
ser is definitely faulty and must be re-
Condensers can be tested off the tractor
on test units. Follow manufacturer's
recommendations to make the following
120 ELECT 13
4. Series resistance
- 14 -
Remove, inspect and regap spark plug to
0.025-inch every 100 hours. Bend only
the outer electrode when setting gap.
Do not sandblast, wire brush, scrape or
otherwise service plug in poor condition
— best results are obtained with a new
Use a spark plug wrench to remove old
plug. Always use a new spark plug gas-
ket when replacing plug. Tighten plug
to 15 to 20 ft. lbs. torque.
Good operating conditions are indicated
if plug has light coating of gray or tan
deposit. A dead white, blistered coating
could indicate overheating.
A black (carbon) coating may indicate an
over-rich fuel mixture caused by clogged
air cleaner or improper carburetor ad-
j us tmen t .
120 ELECT 14
TIMING ENGINE (STATIC TIMING METHOD)
Remove breaker point cover. Remove spark
plug lead to prevent unintentional start-
ing. Rotate engine by hand in direction
of normal rotation (clockwise when viewed
from flywheel end). Points should just
begin to break as the "S" mark appears
in the center of the timing sight hole.
Continue rotating engine until points
reach maximum opening. Measure gap with
feeler gauge — gap should be 0.020-inch
when fully open.
If necessary, loosen point gap adjusting
screw and adjust gap to 0.020-inch. Maxi-
mum gap setting can vary from 0.018 to
0.022-inch, to achieve smoothest running.
Securely tighten adjusting screw after
120 ELECT 15
- 15 -
TIMING ENGINE (WITH TIMING LIGHT)
Several different types of timing lights
are available. Follow manufacturer's
instructions for type used. The follow-
ing timing procedure can be used with
most timing lights.
Connect one timing light lead at the
spark plug terminal.
Connect second timing light lead to po-
sitive side of battery — see timing light
instructions for battery size, wiring, etc.
Connect third timing light lead to ground.
Rotate engine by hand until "S" mark is
visible. Chalk "S" line for easy reading.
The "S" mark indicates the spark position,
which is 20 degrees before top dead center.
Start engine. Run at 1200 to 1800 rpm.
Aim timing light into sight hole. The
light should flash just as "S" mark is
centered in sight hole of blower housing.
If timing is off, remove breaker point
cover, loosen gap adjusting screw and
shift breaker plate until "S" mark is
exactly centered. Re tighten adjusting
screw before replacing breaker point
120 ELECT 16
* * * * *
An alternator system supplies electri-
cal energy to charge the 12-volt battery.
The battery, in turn, furnishes energy
for cranking, lights, and other acces-
The 10-ampere alternator features (1)
a permanent magnetic ring bolted to the
inside rim of the fly-wheel, (2) an al-
ternator stator assembly bolted to the
engine bearing plate and (3) a rectifier-
regulator unit mounted externally on the
120 ELECT 17
- 16 -
The magnetic ring Is composed of 12 per-
manent magnets imbedded in a cast ring.
These high strength magnets are arranged
between pole pieces providing an equal
number of north and south magnetic poles.
The magnetic ring, bolted to the inside
rim of the fly-wheel rotates around the
As the magnetic ring rotates about the sta-
tor, an alternating current (AC) is gener-
ated in the stator winding. This alternating
current is routed to a rectifier-regulator
where it is converted to direct current (DC).
Direct current conversion is required as
the battery will not accept alternating
current because of its electrical polarity.
With this system, the battery and alter-
nator work hand-in-hand to supply the
needs of the engine and accessories —
each one being dependent upon the other.
The a^ltemating c^urrent produced in the
alternator is changed to d^irect c^urrent
in the rectifier- regulator unit. M-
rect c^urrent is necessary for charging
This change is accomplished through the
use of solid state electronic devices
which are arranged to form a full-wave
Regulation is also provided by electronic
devices (Zener diodes) which "sense" the
counter-voltage created by the battery
to control or limit the charging rate.
Since these devices generate heat in
operation, the regulator is equipped
with finned surfaces to provide a greater
When the battery is in a low state of
charge, the regulator permits a higher
charge rate to the battery. When bat-
tery is fully charged, the devices li-
mit the charging rate.
- 17 -
CHARGE CIRCUIT TESTING
When testing the charging system of the
tractor, use the following sequence:
It is Important to inspect for a blown
SfE-lA fuse in the rectifier-regulator
before proceeding to test the charging
Replace fuse and inspect for original
cause of system short or overload.
Start engine. Remove 3-prong connector
from rectifier-regulator unit. Insert
leads from AC voltmeter into the two
outer plug connections. Check voltage
recorded with engine running at full
speed (no load) .
Alternator should supply 28 to 32 volts
to the regulator.
If no charge is being received by the
battery (0 ammeter reading) and voltage
is substantially below 28 volts. the
stator is defective and should be re-
If no charge is being received by the
battery (0 ammeter reading) and voltage
is more than 25 to 28 volts, the stator
is producing properly. Therefore, by
process of elimination, the rectifier-
regulator must be at fault.
120 ELECT 18
DC AMPERES TEST
To check amperage, battery must be in
need of some charge value. If battery
is partially discharged, proceed, if
not, remove spark plug wire and crank
engine 15 seconds to partially dis-
charge battery. Install spark plug
Caution: Never start an engine through
a test meter.
Start the engine and install ammeter in a
series between battery positive cable and
positive battery post.
Adjust polarity on meter. Ampere read-
ing should be from 1 to 10 amps depend-
ing on battery condition. If battery
is partially discharged and no amps are
available, and previous test proved the
alternator to be functioning properly,
the rectifier-regulator is defective.
If DC voltage and amperage prove satis-
factory, there is no reason to service
the charging system if battery will not
stay charged. The problem would be in
Type .. Double-Acting, Pedal-Operated
Parking Hand Lock to Foot Brake
Model No. 15-15 Series
Oil Reservoir Rear Axle Housing
Capacity 5 US Quarts
Lubricant JOHN DEERE Type
mission - Fluid
Forward 1/2 to 6-3/4 mph
Reverse 1/2 to 3-3/4 mph
Remove roll pins which attach drive
shaft yokes to drive shaft. Remove
capscrews that retain the rear yoke to
flexible disk. Slide shaft forward and
remove rear yoke. With the rear yoke
removed it is possible to slip drive
shaft out with ease.
120 PT 1
After the drive shaft has been removed
the flex-coupling can be removed from
the clutch cone by removing the two re-
To service the clutch assembly it is
suggested that the engine and clutch
assembly be removed as an assembly.
This provides for a better inspection
and repair procedure.
Remove engine and clutch assembly by
removing engine mounting bolts, dis-
connecting fuel and electrical wiring
and finally unbolting flex-coupling
from clutch cone. Attach hoist and lift
complete engine assembly forward and up.
Remove clutch assembly from engine by
removing the four attaching capscrews.
Disassemble and inspect internal components,
INTRODUCTION TO HYDROSTATIC DRIVES
The hydrostatic drive is a fluid drive
which uses fluid under pressure to
transmit engine power to the drive
wheels of the tractor.
Mechanical power from the engine is
converted to hydraulic power by a hydro-
static transmission. This power is then
converted back to mechanical power
for the drive wheels.
The hydrostatic drive can function as
both a clutch and transmission. The
final gear train can then be simplified,
with the hydrostatic unit supplying in-
finite speed and torque ranges as well
as reverse speeds.
Only three factors control the operation
of a hydrostatic drive:
Rate of oil flow — gives the speed
Direction of oil flow — gives the
Pressure of oil — gives the torque
Control of these three factors is in-
finite, giving endless selections of
speed and torque in a hydrostatic drive.
120 PT 2
Hydrostatic transmissions use oil at
high pressures but relatively low flow.
Basically, energy is transferred by the
oil itself in a continuous circuit be-
tween the pump and motor as shown.
While the oils does flow through the
lines, it is still considered as being
120 PT 3
Fig, 2 — Basic Types Of Hydrauiic Transmissions
- 21 -
Two cylinders, each containing a piston,
are connected by a line. The cylinders
and the line are filled with oil.
When a force is applied to the left
piston as shown, that piston moves against
the oil. The oil will not compress, so
it acts as a solid and forces out the
In a hydrostatic drive, several pistons
are used to transmit power — one group
in the pump sending power to another
group in the motor.
120 PT 4
The pistons are in a cylinder block and
revolve around a shaft. The pistons
also move in and out of the block para-
llel to the shaft.
Shown is one piston from the pump and
one from the motor. To provide a pump-
ing action for the pistons, a plate
called a swashplate is located in both
the pump and motor. The pistons ride
against the swashplates.
The angle of the swashplates can be
varied so that the volume flow of the
oil pumped by the pistons can be changed,
Adjusting the swashplate affects flows
and speeds .
120 PT 5
'"is- 6 Two Connected Cylinders Wiih Swashplates
- 22 -
A pump or motor with a movable swash-
plate is called a variable-displacement
unit. A pump or motor with a fixed
swashplate is called a fixed displace-
As you have already learned, pumps and
motors can have a fixed displacement
or variable displacement.
Four pump-motor combinations are possible:
1) Fixed displacement pump driving a
fixed displacement motor.
2) Variable displacement pump driving a
fixed displacement motor.
3) Fixed displacement pump driving a
variable displacement motor.
4) Variable displacement pump driving a
variable displacement motor.
120 FT 6
OPERATION OF HYIHIOSTATIC
120 PT 7
Shown is a variable displacement pump
driving a fixed displacement motor as
is used in the 120 Tractor.
As the pump pistons rotate, they move
across the sloping face of the swash-
plate, sliding in and out of their cyl-
inder bores to pump oil in and out.
The more the pump swashplate is tilted,
the more oil it pumps with each piston
stroke and the faster it drives the
The motor swashplate is at a fixed angle
so that the stroke of its pistons are
always the same. Thus its speed of
rotation cannot be changed except as it
is driven faster or slower by the pump
The point to remember now is that a
given volume of oil forced out of the
pump at a given pressure will cause the
motor to tutn at a given speed.- More
oil will increase the motor speed. Less
oil will slow it down.
- 23 -
The pump is driven by the tractor engine
and so Is linked to the speed set by the
operator. It pumps a stream of high-
pressure oil to the motor.
Since the motor is linked to the drive
wheels of the tractor, it gives the
tractor its travel speed.
Since the pump flow is variable the
output speed is variable. At any given
pressure, output torque is constant.
The pump-motor combination is the heart
of the hydrostatic drive, although the
complete hydraulic system also includes
a reservoir to supply the oil, a filter
to remove dirt, and a cooler to remove
excess heat from the oil.
Basically the pump and motor are joined
in a closed hydraulic loop; the lines
of the motor are joined directly to the
pump. A charge pump supplies cooling
and makeup oil for closed loop as well
as implement oil.
120 PT 8
Fig. 9 — Pump-Mo'or Form A CloseU HydrauMc Loop
REVERSING IN VARIABLE PUMP CIRCUITS
The direction of the motor output shaft
rotation can be reversed by shifting the
pump swashplate over center .
NOTE: Pump input shaft always rotates one
In neutral, the swashplate is vertical
and no oil is pumped.
In forward, the swashplate is tilted and
oil is pumped as shown at the top.
In reverse, the swashplate is tilted the
opposite way and the unit pumps oil in
the opposite direction.
120 PT 9
- 24 -
Shown is the circuit diagram of the hy-
drostatic used in the 120 Tractor.
In order to keep the closed loop circuit
full of oil a gerotor-type charge pump
Oil is supplied by the charge pump to
the low pressure side of the circuit
through one of the check valves.
The high pressure oil on the high pres-
sure side holds one check valve closed.
The charge pressure oil on the other
side is allowed to enter the low pressure
side of the circuit, through the check
valve, as needed.
A pressure of 75 to 110 PSI is maintained
by the charge relief valve. This valve
also allows excess oil not needed by the
closed loop to pass into the hydraulic
lift circuit. The oil then continues
through the lines to the open center
valve and returns at low pressure through
the pump case to the reservoir.
The hydraulic lift circuit is protected
from oil pressure damage by the imple-
ment relief valve. Whenever pressure
in the implement circuit exceeds approxi-
mately 500 PSI, the valve opens, allowing
oil to by-pass the implement circuit and
flow into the case drain.
The hydraulic system has controlled in-
ternal oil flow, which lubricates cer-
tain critical areas in the unit.
Again, remember the charge pump supplies
cooling and makeup oil as well as imple-
120 PT 10
- 25 -
Remove the fender deck assembly by re-
moving the (2) bolts in the front, (one
on each side) and remove the nuts on the
rear two rubber mounts .
Remove the knobs on the mower depth stop,
free wheeling control and park brake con-
NOTE: If tractor is equipped with lights,
disconnect the taillight wires.
120 PT 11
Remove the shield covering the hydro-
static unit and the two oil lines (top
front) leading to the implement circuit.
Remove the nut holding the speed control
rod to the hydrostatic cam.
Drive the roll pin out of the driveshaft-
inputshaft coupling or disconnect the
two bolts holding the yoke to the belt
120 PT 12
Drive out roll pin holding fan on the
rear of the hydrostatic and remove fan
and fan guard.
120 PT 13
Remove the bolts holding the transmission
shield. Remove the rear transmission
supports. Remove the tension spring on
the hydraulic control lever.
120 PT 14
- 26 -
Block the tractor frame up to support
tractor. Remove the (4) bolts holding
the hitch plate to the frame. Discon-
nect the brake connection.
120 PT 15
Roll the transmission and differential
assembly away from the tractor. Discon-
nect the large oil cooling line and drain
the oil. Remove the (4) bolts holding
the transmission to the axle assembly.
The hydrostatic is now ready for internal
120 PT 16
*' ^ nB£_
Disassemble the hydrostatic as follows:
TESTING AND DISASSEMBLY
When experiencing problems with the trac-
tor operation, always check the manual
linkages before determining an internal
hydrostatic component is causing the trouble.
Charge Pump Relief Valve:
The charge pump relief valve may be ser-
viced without removing the transmission.
Correct charge pressure is 75-110 PSI.
This may be checked by tapping into the
center section and connecting a pressure gauge,
Be sure the hydrostatic is in neutral for
If replacing the charge relief valve as-
sembly, you notice shims on the old as-
setably, check the charge pressure after
the new assembly has been installed. Due
to tollerences in the machined parts in the
center section, it may be necessary to add
shims to get the proper pressure reading.
120 PT 17
- 27 -
Remove the charge relief valve plug, spring
and cone from the center section of the
Wash and dry the components. Check the
spring for pitting or rust. Check the
cone for wear or damage. Check the seat
in the center section of the housing for
nicks, dirt or scratches.
Install the relief valve in the center
section as shown. Install a new 0-ring
when reassembling. Torque plug 15-17 ft.
120 PT 18
Symptoms of defective charge relief
1, Won' t move,
2, Erratic or jerky,
3, Lack of implement control.
* * *
Charge Pump Service:
Caution: Thoroughly clean and deburr
all shafts and housing before disassem-
Remove the two hexs crews holding the
charge pump to the center section hous-
ing. Carefully remove the pump housing
as the rotor assembly may stick to the
housing. Do not drop the assembly. Re-
move the rotor assembly if it did not
come off with the housing.
Note the method of retaining' the rotor
gear with the rotor pin.
The charge pump housing has a seal and
0-ring, which are replaceable.
Thoroughly libricate all parts with
hydraulic oil before assembling (JD303) ,
120 PT 19
- 28 -
Reassemble rotor gear to pump shaft and
secure with rotor pin. Install roller
in the charge pump housing. Install hous-
ing with flat edge to the charge relief
valve side. The charge pump assembly
is serviced as an assembly only. Torque
the pump housing cap screws 50-55 ft. lbs.
Symptoms of Defective Charge Pump:
1. Erratic operation.
2. Lack of operation.
3 . Won ' t move .
4. Lack of implement pressure.
120 PT 20
Implement Relief Valve:
Remove implement relief valve plug, spring
and cone from the center section of the
housing. Wash and dry the components.
Check the spring for pitting or rust.
Check the cone for wear or damage. Check
the seat in the center section of the
housing for nicks, dirt or scratches.
Replace the relief valve in the center
section as shown. Torque 15-17 ft. lbs.
Symptom of defective implement relief
valve could be lack of implement pressure,
however, the charge pump relief valve or
charge pump may be causing the problem.
Implement pressure may be checked by
attaching a gauge to the front outlets.
Proper pressure is 500-550 PSI.
NOTE: If shims were on the old valve,
it may be necessary to check pressure
and add shims to the new relief valve to
arrive at the proper implement pressure.
120 PT 21
- 29 -
Again, it is not necessary to remove the
transmission from the tractor to service
the check valve.
A quick external check of the check valves
would be to push the pins in the center
of the plugs down (with the engine running)
and observe to see if they rise rapidly.
It may still be necessary to remove the
valves to be sure of their condition.
120 FT 22
Remove the check valve from the center
housing. Wash valves in clean solvent
and air dry. These valves are completely
Check valves for dirt, corrosion and free
The valves are serviced as an assembly
only. Replace the valve if in doubtful
condition. Always replace the 0-ring
and back-up washer when reassembling
check valves. These parts are serviced
separately from the check valves.
Lubricate all parts with hydraulic fluid
and reinstall in center housing section.
1. Won't move one direction or
2. Erratic (maybe).
120 PT 23
Be sure the outside surfaces of the
transmission have been thoroughly cleaned.
Place the transmission assembly in the
Remove the charge pump assembly. Remove
the oil filter. The oil filter union is
to be 1/2-inch from the filter mounting
Ti^D n M
- 30 -
Remove the cap screws securing the cen-
ter section housing to the transmission
housing. Torque bolts 33-37 ft. lbs. on
Lift the center section housing from the
IMPORTANT: The valve plates may stick
to the center section housing surface.
Be extremely careful not to drop them.
Remove the pump and motor valve plates
(if not removed with center section)
noting the location of each plate. The
valve plate with two notches is used on
the pump assembly and the plate with
four notches on the motor assembly.
Thoroughly clean the valve plate and
blow dry with compressed air.
Inspect the valve plate for scratches,
excessive wear or erosion. A worn or
scored valve plate reduces pump effi-
NOTE: To check the plate for wear, run
your finger nail or a sharp pencil across
the face of the plate. If wear is felt,
replace the plate.
120 PT 25
Inspect the pin slot and grooves of the
valve plate. Clean out any foreign mat-
ter and deburr the surface as necessary.
- 31 -
The center section needle bearings are
The new center section needle bearings
must be installed so that they extend
. 100-inch above the machined surface of
the center section. The bearings "pilot"
the valve plates when the unit is re-
Then ends of the bearings with the print-
ing go to the outside.
Tip the transmission housing upside down
holding the pump and motor cylinder
block assemblies so they do not fall out.
Holding the housing and one cylinder
block assembly, remove the other cylin-
der assembly. Then, remove the remaining
cylinder block assembly. Do not let the
piston fall out and get damaged. Do not
mix the motor and pump pistons.
120 PT 2 7
120 PT 2
Remove the pistons from the slipper re-
tainer. Thoroughly clean the pistons
and blow dry with compressed air. Be
certain center oil passage is open.
T2n PT ?Q
- 32 -
Carefully inspect each piston for scor-
ing, wear or scratches.
Check the slippers for severe scratches or
embedded material. Slippers may be lapped,
but do not remove more than .005 inch.
All slippers must be within .002 inch
thickness of each other.
Replace any pistons that are badly worn
Inspect the slipper retainer for damage.
A slight wear pattern where the slippers
ride is normal. Replace if wear is ex-
120 PT 30
To disassemble the cylinder block it is
necessary to compress the spring to re-
move the retaining ring.
A long bolt can be used with washers on
each end to compress spring.
CAUTION: Do not over tighten bolt as this
will damage the spring retainer.
Remove the retaining ring.
120 PT 31
Carefully remove the bolt releasing the
spring pressure. Remove the spring re-
tainer, spring and spring washer.
Thoroughly clean all parts and blow dry
with compressed air.
Check the spring against specifications.
The spring in the piston block holds
pressure against the valve plate.
Check the cylinder block valve face for
damage and the piston bushings for ex-
cessive wear. Any linear scratches
along the length of the bore will re-
duce efficiency. Check piston fit in
Compress the spring, using the method
explained above. Install the retaining
ring. Release the spring pressure.
120 PT 32
- 33 -
Remove the cap screws securing the motor
Torque screws 60-70-inch lbs. on reas-
120 PT 33
Remove the transmission motor shaft and
swash plate from the housing.
120 PT 34
The motor shaft bearing is to extend
.18-inch from the housing.
120 PT 35
- 34 -
Remove the trunnion shaft assemblies
from the hydraulic pump swash plate by
driving on the spring pins. The pins
will bottom against the trunnion and
will be centered in the trunnion shaft.
Remove the thrust plate and the swash
plate from the pump section.
Be sure to install the pump swash plate
with the thin pad toward the top of the
Inspect the thrust plate (for the pump
swash plate) for wear, embedded material,
Inspect all the bearings and replace as
120 PT 37
The trunnion bearings are to be installed
flush or .005-inch below the inside sur-
face of the housing.
120 PT 38
- 35 -
Pump shaft and trunnion shaft seals are
to be flush or .005-Inch below the out-
side surface of the housing.
The seals may be replaced without dis-
assembling the hydrostatic.
120 PT 39
Reassembly is basically the reverse of
disassembly however, particular attention
should be given the following:
Thoroughly lubricate all parts in clean
Pipe plugs or implement circuit fittings
in the center section housing showing
leakage must be removed, doped (teflon
tape on the threads is ideal) and rein-
120 PT 40
Use all new 0- rings , seals and gaskets.
Be sure the thin pad of the pump swash
plate is toward the top of the trans-
mission housing. Seat the spring pins
only flush with the trunnion as shown.
Be careful not to drop piston assemblies
upon reassembly. Hold the unit as shown.
120 PT 41
- 36 -
Be sure the notches of the valve plates
are in place over the studs in the center
section. Grease the plates lightly to
hold them during reassembly.
120 PT 42
Hold the unit on its side as shown for
assembly. Be careful valve plates do
not slip out of place.
The springs in the cylinder blocks will
hold the case and center section apart
about 1/4 to 1/2 inch. This distance
can be compressed by hand to be sure
pieces are fitting properly. Replace
the bolts holding the two sections to-
* * *
120 PT 44
Brake service may be performed without
removing differential from the tractor.
The wheel and hub being removed must be
blocked up 6 to 8 inches to keep the
oil from running out the axle tube.
Remove the right rear wheel and bolt
from the center of the hub. Replace the
bolt in the hub without the flat washer.
Use the puller as shown to remove the
brake drum from the axle.
With the drum removed, disconnect the
(4) springs holding the brake shoes in
place. Be careful not to cut the spring
during removal and replacement.
120 PT 45
- 37 -
If it was necessary to remove the (4)
bolts holding back plate to seal retainer,
it will be necessary to use a tool such
as the one shox>m to center the retainer
and seal with the axle shaft to get a
good seal seat. The seal seats on the
axle hub and brake drum surface.
The tool is made from a piece of tubing
with an outside diameter of 1.500 and
an inside diameter of 1.130. The tool
is also available from Service Tools,
Inc., 1901 Indiana Avenue, Chicago, Illi-
nois 60610 Part number JDST-9.
120 PT 46
Inspect the brake shoes for wear, dirt
or oil contamination. If oil seal has
been leaking, it may be necessary to re-
place the shoes due to oil contamination.
Install the shoes and (4) retaining
springs. Be sure that the brake shoes
fit into the rubber retainer on each end.
Install the brake drum using a 7/16"x3"
or longer NF bolt to draw axle shaft and
brake drum together.
Install the nut about half way up the
bolt. Place a large flat washer over
the bolt and turn the bolt into the axle
Hold the head of the bolt and draw axle
and drum together as shown by turning the
Do not try to drive the drum on the axle
as this will push the bearing off the
axle. The bearing has no snap ring or
retainer ring. The bearing is a press
fit on the axle.
Make sure that the brake drum is not
catching on the brake shoe during in-
120 PT 47
- 38 -
Remove the bolt used to install the drum
and reinstall the bolt and washer, which
was originally removed from assembly.
Torque the bolt 30 to 40 ft. lbs.
AXLE SHAFT, SEAL OR BEARING SERVICE
Remove the rear wheel, axle hub or brake
drum as was explained in the brake ser-
vice section. It is not necessary to
remove the differential from the tractor.
Remove the (4) bolts holding the oil seal
and retainer plate on the axle tube. No-
tice the position of the retainer plate
as it is removed. The rounded surface of
the retainer plate goes to the outside
next to the seal.
120 PT 48
Pull the axle shaft out of the extension
Note the 0-ring on the shaft. If the
0-ring is removed from the shaft, it will
be necessary to install a new 0-ring.
This 0-ring seats against the wheel hub.
The bearing on the axle shaft can be re-
placed by pressing it off the shaft as
When installing the new bearing, be sure
to press on the inner race only. Seat
the bearing tight against the shoulder.
Be sure to install a new 0-ring on the
The cup may come loose from the bearing.
If this occurs, clean the bearing and cup
and reinstall in the axle tube.
120 PT 50
- 39 -
Slide the axle shaft in the extension
tube. Install the gasket, the retainer
plate (rounded side out) , a gasket and
the oil seal. Secure loosely with the
Use the seal tool (as explained in the
brake service section) to center the oil
seal with the axle shaft. This is im-
portant, because if the seal is not cen-
tered, it will leak oil.
Tighten the (4) nuts 15-30 ft. lbs. The
torque is important as it clamps the
bearing in position.
120 PT 46
Install the hub back on the shaft. Use
the 7/16" NF bolt as explained in the
brake section to reinstall the hub on
Do not hammer hub on the shaft as it is
possible to drive the bearing off the
Torque the center bolt 30 to 40 ft. lbs.
Do not reuse the bolt if it has lost its
locking feature .
The differential assembly may be removed
in the same manner as explained for hy-
After rolling the transmission and dif-
ferential assembly away from the tractor,
remove the hydrostatic and the wheels.
Remove the hitch plate and drain the
oil out of the axle.
120 PT 16
- 40 -
With the differential assembly removed
from the tractor, remove the hubs and
axle shafts as previously described.
Note: The axle shafts must be removed
to be able to disassemble axle housing.
Remove the (8) screws that hold the
front housing to the rear housing. (The
rear housing is the half with the hitch
plate welded on.) There are two dowel
pins connecting the case halves. Do
not move or remove these dowels.
120 PT 52
****5V5': ;>;****** >'<*******
With the front housing removed, look for
the identification mark on the bearing
caps and the case housing. The bearing
caps are matched. Do not interchange
them. Some early models did not have
match marks and it is necessary to mark
the caps for reassembly as shown. Also,
note the position of the differential
case in the bearing cradles. The long
side of the differential assembly
goes toward the side cover plate.
Remove the (4) bolts that hold the bear-
ing caps in place. Lift the differential
assembly from the bearing cradles . If
the differential sticks in the cradle,
use two pieces of wood or hammer handles
to pry the unit out.
Do not interchange the bearing races.
120 PT 53
Disassemble the differential assembly
by driving out the roll pin as shown.
120 PT 54
- 41 -
* * * *
With the roll pin removed, use a punch
to drive the pinion shaft out of the
case. Be careful not to hit the ring
gear while removing the roll pin or pin-
120 PT 55
Rotate the pinion gears as shown to re-
move them from the case. Remove the
differential drive gears.
If the pinion gears do not remove easily,
use an axle shaft to turn the gears out.
120 PT 56
The ring gear may be replaced by removing
the (10) bolts holding it in place. Use
a block of wood to knock the old gear off
the case to prevent damage to the hous-
ing. Clean the housing surface and in-
stall the new ring gear and torque the
bolts 45-60 ft. lbs. in a crisscross
pattern. This will provide a uniform
seat of the gear.
120 PT 57
- 42 -
Install the thrust washers on the dif-
ferential drive gear hubs. Grease the
hubs and washers well and insert the
gears into the differential case as
120 PT 58
Grease the pinion gear and thrust washer.
Align the pinion gears and washer di-
rectly opposite each other. Roll pin-
ion gears in place by turning the dif-
ferential drive gears until the holes
line up. Be sure holes are directly
across from each other on installation.
Use a punch to be sure washer and pinion
gears are lined up with the differential
120 PT 59
Install the pinion shaft into case and
pinions, being careful to line up roll
pin hole in shaft and differential case.
Drive roll pin into case and shaft from
the long side of the case. Stake the
roll pin in place. Be careful not to hit
the ring gear with the hammer or punch.
A noisy differential can be caused by hitting
the ring gear with a hammer.
120 PT 60
- 43 -
The differential bearings are replace-
able. Remove them with a puller as shown.
Note there are two grooves in the case
to facilitate pulling.
Note the shims under the bearings. These
shims provide the backlash adjustment on
the differential. The puller may mark
the shims ,
If only one bearing is to be replaced
use the same shims that are under the
If both bearings are to be removed, do
not interchange the shims from side to
If new bearings are being installed on
a new case, start with .020 shim on
120 PT 61
Install the new bearing by pressing on
the inner race only. Turn the shims un-
der the bearing so the edge that was
marked by the puller is in alignment with
the notches in the housing. Seat bearing
120 PT 62
To remove the spur gear, remove the
side cover plate.
120 PT 63
- 44 -
To remove the pinion assembly, drive a
small punch into the cover of the welch
plug about 3/8" from the edge. Pry the
cap from the housing.
120 PT 64
Remove the snap ring and spacer from
the pinion shaft.
120 PT 6 5
Rest the housing on a block of wood on
the press bed so as not to damage the
bearing cradle. Press the pinion out
of the housing as shown. Place a screw-
driver tip or about 1/8" material under
the edge of the spur gear during removal
of pinion shaft. This will eliminate the
spur gear getting in a bind.
120 PT 66
- 45 -
This is the complete pinion and spur
The bearing may be removed from the
pinion shaft using an adjustable press
plate and a hydraulic press.
Press the new bearing on the shaft by
pushing on the inner race only.
120 PT 67
The outer bearing race may be removed
and replaced as shown.
120 PT 6
The inner race is also replaceable, but
must be pressed to the outside.
There are shims under this bearing race
which are used to set the ring gear and
pinion tooth engagement.
120 PT 69
- 46 -
ASSEMBLING PINION AND DIFFERENTIAL GROUP
There are two adjustments which must be
made when reassembling the axle. Ring
gear and pinion tooth pattern (1) and
backlash (2) .
The ring gear and pinion tooth pattern
must be set before checking the backlash.
The ring gear and pinion tooth pattern
can be changed by adding or removing
shims located under the inner bearing
race. More shims will increase tooth
engagement and less shims will decrease
If when servicing the axle, the old
housing is to be reused, use the same
shims under the bearing race.
If a new housing is to be used or for
some other reason it is not possible to
reuse the old shims, determine the shim
pack by measuring as shown.
Using a press, install the bearing cup
in the housing without any shims. Place
the pinion assembly into the cup.
Use the special tool JDST-10 as shown
with a feeler gauge to determine the shim
pack required. The thickness of the feeler
gauge will determine the shim pack to be
installed under the bearing cup.
Remove the bearing cup, install the shims
and replace bearing cup.
- hi -
Install the pinion and spur gear in the
housing as shown. Install the spacer
with the flange over the splines. (Down)
The spur gear is interchangeable.
120 PT 71
Press the outer bearing into position.
Support the pinion shaft which the
bearing is being pressed on. Press on
the inner race only until the cup and
cone are seated.
Rotate the head of the pinion. There
should be a slight amount of drag on the
pinion. If it is too tight, tap on the
outer end of the pinion shaft.
Install the spacer washer and snap ring
on the end of the pinion shaft. Spacers
of various thicknesses are available if
the original does not fit.
120 PT 72
Install the cups on the bearings and
place the differential assembly in the
bearing cradles. The differential should
fit snuggly in the cradles. Use a rubber
mallet to seat the differential assembly
in the cradles. If the fit is too tight,
it may be necessary to remove some shims
under the case bearings.
Be sure teeth on ring gear and pinion
are aligned when seating the differential
in the cradles. The long side of the
differential case points to the side
Install the bearing caps according to
match marks and torque the bolts 30-45
120 PT 73
- 48 -
Paint the teeth of the ring gear with
marking compound and turn the differen-
tial a few seconds.
Check the pattern of ring gear and pin-
ion mesh. If a toe picture is present,
move the pinion out. If a heel condi-
tion is present, move the pinion in.
Shown is a sample of various types of
wear patterns .
Note the portion of the ring gear called
the toe and heel. Also note the side of
the tooth called the drive side and
coast side. The most accurate ring gear
pattern will be seen on the drive side.
Note the wear pattern in the center of
the tooth. This is the preferred pattern.
This could result in a noisy gear set.
Move the pinion gear away (out) from the
This could result in a noisy gear set
and possibly early failure. Move the
pinion toward (in) the ring gear.
Move the ring gear toward the pinion
to change this condition. (Backlash
Ad jus tment)
Move the ring gear away from the pinion
to change this condition. (Backlash Ad-
120 PT 74
- 49 -
Check the backlash with a dial indicator
as shown. Backlash range is .003 to
.007. If the backlash is not in this
range, shims must be moved from side to
side on the differential case bearings.
If backlash is too tight, move ring gear
away from the pinion. If it is too loose,
move the ring gear toward the pinion.
/■ ' — - — >■
Install a new welch plug with the lip
pointing in. Be sure the plug is straight
when installing. Install the side cover
and gasket. Be careful not to over
tighten these side cover screws.
Bolt the rear housing on the front hous-
ing and torque the bolts 18-23 ft. lbs.
Replace axle shafts and seals (15-30 ft.
lbs.) and hubs (30-40 ft. lbs.) as ex-
Reassemble differential assembly in the
reverse procedure from installation.
120 PT 52
- 50 -
The hydraulics on the 120 are quite simi-
lar to the 140 HI Tractor.
All items concerning external hydraulics
are covered in the 120 and 140 Service
* * >v
^ -k •^i -k
The miscellaneous items, such as steering,
front axles, tie rods and spindles are
covered in the 120 and 140 Service Manuals
quite well. No further discussion will be
made in this slide text.